Black oxide coatings on coarse iron castings



United States Patent ()ffice US. Cl. 204-35 14 Claims ABSTRACT OF THE DISCLOSURE A method of producing a blue-black iron oxide finish on a cast iron or compressed iron-powder article by electroplating an iron coating onto the article from an alkaline iron plating bath having a pH of from 11 to 13.8 and containing an iron sequestering agent, and thereafter oxidizing the coating to obtain a blue-black iron oixde coating.

This invention is that of a method of producing a blueblack iron oxide coating over a cast iron (such as nodular iron, gray or white cast iron, and malleable iron) by cleaning from the ordinarily coarse surface of a cast iron, e.g. nodular iron, any soil which might prevent adherence of a fresh electrodeposit on it, electroplating over the cleaned surface from an aqueous alkaline plating bath containing chelated iron a firmly adherent covering electrodeposit of iron, removing adhering plating solution from the deposit, and then subjecting the iron electroplated surface to a customary treatment for the blueing, i.e. producing a blue-black iron oxide coating, over a clean steel surface.

The cost of producing various devices which include metal parts which have cavities and bores for enabling attaching other necessary parts is unduly high [because those receiving parts are made of steel and production of the cavities and bores in it requires considerable hand labor. Significant savings could be made if such parts be cast and then be given a suitable finish consistent with that of the other parts assembled with them to provide the finished device.

For example, rifles used by infantry soldiers could show a desirable production saving if their receivers (with bores for engaging the rifle barrel and butt, and cavities for holding the firing pin, trigger and operating mechanism) could be cast with their required bores and cavities. It was found that suitable castings of these receivers could be made from a cast iron such as nodular iron. Adoption of receivers made of a cast iron particularly nodular iron was retarded because of difficulties in finishing their outer surfaces.

Various attempts to apply suitable finishes were made, but each had its specific shortcomings. A desirable consideration was to attempt to produce on these cast receivers a blue black iron oxide coating like that on the rifle barrels. However, that was Without success because a suitably satisfactory blue-black oxide coating could not be produced even on the buffed and polished nodular iron receiver.

As part of the development leading to the invention, attempts were made to attain a satisfactory blue-black iron oxide coating by intermediate electroplating of iron over he buffed and polished receivers. Unsaisfacttory coatings with red spots, stains and/or peeling due to poor adhesion resulted.

The various shortcomings were overcome and uniformly satisfactory blue-black oxide coatings resulted from the process of the invention.

Broadly considered the invention embraces the method 3,496,074 Patented Feb. 17, 1970 of attaining a. blue-black iron oxide coating over a cast iron article as well as on a compressed and sintered iron powder article by cleaning from the article any soil (including rust) which might prevent a fresh electrodeposit on it, immersing the article in an aqueous alkaline plating bath having a pH significantly over 11, and advantageously from about 12.8 to about 13.8 electrometric, and containing chelated iron in a concentration sutficient to avoid exhaustion of iron during the plating operation, and passing a direct electric current from an anode, advantageously comprising iron, through that plating bath held at an elevated temperature to the article as a cathode at a current density and for a time sufiicient to provide a firmly adhering electrodeposit of iron thick enough to enable attaining on it a substantially uniform blue-black iron oxide film without destruction of the continuity or adherence of the electrodeposit, removing adhering plating solution from the electrodeposit, and then subjecting that iron electroplated surface to a known treatment to produce a blue-black iron oxide film over a clean steel surface.

While the invention can be carried out on any of the different forms of cast iron and sintered compressed powdered iron articles, initially it is particularly applicable to rifle receivers. Thus, the invention may be described with particular reference to preparing a blue-black iron oxide coating on them without any intention to disregard its broader application as already expressed. With the rifle receivers, it is desirable that initially their castings be buffed and polished.

Then, before electroplating iron on them, any soil, inincluding any rust, buffing and polishing compounds, should be removed to avoid the adverse effect of any of them on uniform application and adherence of the electrodeposit.

Where necessary, the soil removal can be done separately by any of the respectively suitable available methods, depending on the nature and extent of the soil. In many cases, wherein any residual molding sand, rust', or buffing and polishing compound residue are not extraordinarily excessive, such soil can be removed in the same alkaline bath in which the iron electrodeposition is to be carried out.

Effective as such iron plating bath for the firmly adhering electrodeposit of iron is any of a wide variety of aqueous alkaline baths having dissolved therein various effective amounts of one or more sequestering agents which form a water-soluble chelate with iron, whether ferrous or ferric, along with an effective concentration of an alkali metal hydroxide, such as sodium or potassium hydroxide or ammonium hydroxide.

Particularly effective sequestering agents are the various monohydroxy or polyhydroxy, monoor polycarboxy lower aliphatic acids having from two through seven carbon atoms such as gluconic acid, citric acid, tartaric acid, glucoheptonic acid, and its isomers galactoheptonic acid, fructoheptonic acid, and the mixed hexahydroxyheptoic acids, and saccharic acid, or an amino polyhydroxy lower aliphatic acid such as 3-amino-2,4,5,6,7-pentahydroxyheptoic acid, or other sugar acid, as well as the alkali metal and ammonium salts of any of those acids and the alkaline earth (including magnesium with them; and at least for ferric iron) salts of any of those polycarboxylic acids.

Any of these various six or seven carbon atom polyhydroxy acids or any of the sugar acids can be admixed with one another or with any of the other sequestering agents, and advantageously with from about one-third to three times its quantity of a hexitol such as sorbitol or mannitol.

Additional applicable sequestering agents includes the 3 p ya yl n po y i Po y-a t a id. comp u a d their than) snidrvaienrmai saltsffor example, diethylenetriamine pentaaceticf acid and any of its alkali metal and ammonium salts or even any of its alkaline eaith salts (at least for ferric iron) as its calcium or magnesium salts, and any of the mono-hydroxyethyltetracarboxymethyl diethylenetriamines or dihydroxy ethyl-tricarboxymethyl diethylenetriamines, and any of the corresponding same salts of any of them, as well as any of the corresponding same salts of any of them, as well as any of the free acid and salt form sequestering agents disclosed in US. Letters Patent 2,831,885, 2,848,- 469, 2,859,104 and 2,906,762.

While'individual plating baths can be prepared using any one of the foregoing'and other effective sequestering agents along with a suitable amount of alkali metal hydroxide to give an etfec'tive pH value beneficially over '11, more than one of any of the applicable sequestering agents can be used. There can be included various amounts of ethylenediamine' tetraacetic acid or of any or -its;r'nono'-' to tetra-alkali metal or ammonium salts as well as any of its alkaline earth metal salts (includingmagnesium among them), and generally to the extent of no more than about one-half the amount of the other sequestering agent, or of a lower alkanolamine such as mono-, di-, or triethanolamine and like propanolamines.

Any of the applicable aqueous alkaline baths can in clude an inorganic sequestering agent such as any of the sequestering phosphates such as the alkali metal hexametaphosphates, or some other alkaline phosphate such as a di-(alkali metal or ammonium) hydrogen phosphate as disodium or dipotassium hydrogen phosphate.

The aqueous alkaline baths can be used at any suitable pH beneficially above 11 depending on the sequestering agent or agents and the nature of any other alkaline agent or agents included, and up to about pH 13.8 and possibly somewhat higher. These various alkaline baths are advantageous in exhibiting desirably good throwing power, and serve eifectively in the preliminary cleaning as referred to above.

When the preliminary cleaning is not going to be conducted in the plating bath and also when the amount of rust to be removed is low, the aqueous alkaline plating bath should contain in solution at least a sutficient amount of a compatible iron salt or chelate, which is soluble at the pH of the plating bath to avoid exhaustion of iron during the plating if no iron-bearing anode is used. It is advantageous, however, to use some suitable iron-bearing anode. Thus generally, the starting composition of the bath need contain only very little of a water-soluble iron compound such as an iron salt or chelate, when the bath will be used to clean a sufficient amount of rust from the cast iron articles which are to be given the iron electrodeposit or when iron-bearing anodes are to be used. 1

To provide the initial iron content of the bath when iron-bearing anodes are used, it can contain a relatively small amount such as about one-tenth percent of such water-soluble iron salt as a ferrous or a ferric salt soluble at the pH of the bath, such as ferrous or ferric sulfate, or nitrate, and as thus far noted to be advantageous ferric acetate, as well as any of the iron chelates of any of the applicable sequestering agents and soluble at the pH of the bath at least to the extent sufiicient to enable electrodeposition of iron to be accomplished therefrom.

In some cases, initial content of such iron salt or chelate can he quite small and at times avoided when the bath is to he used for the preliminary cleaning of the cast, e.g. nodular, iron article which is to receive the iron electrodeposit. That is so because such preliminary cleaning, carried out by reverse current procedure, results in removing rust from the article and thus providing an adequate initial amount of dissolved iron in the bath suflicient to enable electrodeposition of iron on the cathode to progress with some dissolution of .iron from the particular iron-containing anode used to enable depositing the necessary firmly adhering electrodeposit of iron on the cast, e.g. nodular, iron article cathode.

For regularly dependable deposition of iron, the total dissolved solids in the bath can range from about one to about four pounds per gallon (i.e. about 115 to about 450 grams per liter). A generally good practical concentration, bearing in mind such factors as conductivity, plating rate, and dragout, is in the neighborhood of about two to about 2.5 pounds per gallon. However, for higher conductivity with certain solutions (e.g. sodium gluconate without any other added salts), it is more desirable to work with solutions of at least two pounds and nearer about four pounds per gallon.

An advantage provided by using an aqueous alkaline plating bath is that an acid-resistant tank. is not needed. The'concentration of the sequestering agent, whether one or a mixture of them, can vary widely, generally from about two to about one hundred percent of the total solids content,and preferabl from about five to about ninety-five percent of it, depending on providing the required pH value or range.

Gray cast iron, black iron or other cast iron may be satisfactory as. anodes of ferrous material to contribute some iron to the bath, from which is obtained a consistently uniform iron electrodeposit on the initially ordinarily coarse-surfaced ferrous metal cathode. Electrolytic iron anodes also are suitable. Even cold rolled steel anodes can be used. To avoid interference with the quality of the iron deposit by suspended carbon particles released from a high carbon content cast iron anode over continued use, it is desirable to enclose such anodes in Orlon or other suitable anode bags, as preferable over periodic or continuous filtration of the bath.

Consideration should be given to the relationship of anode area to that of the articles being plated and thus serving as cathodes. Generally, it is advisable that the anode area be significantly greater than that of the part to be plated, and even up to double its area particularly if the cathode part has deep hollows.

The bath may be operated over a wide temperature range, even as low as ambient (i.e. room) temperature, but at such level the plating rate is very slow (e.g. at about 80 F and the voltage needed for suitable current density is excessive, being from 12 to 15 volts or even more. A presently indicated most practical temperature range is from about 140 to about 180 F., although there is no discernible difference in the adhesion and generally desirable character of the iron electrodeposit even at the lower temperatures. Where conditions permit, very satisfactory practical results occur at as high as 190 F. and

' are obtained even at 200 F. and can be obtained also at I and under many conditions can be as high as 100 amperes per square foot. However, for cathode articles having sharp points or projections, it may be advisible to operate somewhat under amperes per square foot to avoid burning. Ordinarily the lower the temperature, the higher should be the current density.

Thus, the maximum current density for any particular bath should be just under that at which the electrodeposit would begin to show signs of burning. However, the current density generally would have to be Well over amperes per square foot before any indication of burning or other undesirable injury can occur tothe iron electrodeposit on the original cast iron article cathode, or a flaky (and thus undesirable) electrodeposit can be produced.

Electrodeposition time has to vary with the character of the surface to be electroplated, bath composition, temperature, current density, and any other plating condition. With some combinations of conditions, possibly as little as about seven minutes or so may be adequate, and in others more than that and possibly up to about fifteen, or even to about twenty-five minutes or more plating cycle may be needed; the rougher the surface, the lower the temperature, the longer the time required ordinarily. Also the bulkier or heavier the work piece the longer the plating time-even up to about forty-five minutes on, for example, the larger receivers for rifles.

The method of the invention is operable readily in quantity production scale. Quite often the alkaline bath used for the plating step of the method of the invention can be used for the preliminary treatment to remove the average ordinary amounts of soil and rust encountered on the general run of articles which will need to be iron plated in the bath. As already indicated, such soil can be removed by subjecting the articles to preliminary electrolytic treatment, including reverse current, for a time Sllfilcient to remove the soil and rust. That will depend on the type and extent of soil and rust, the bath, and the treatment.

For some combinations of these conditions, including mild soil and/or rust, two or three minutes of reverse current treatment may be adequate. Slightly heavy soil and rust, possibly may need from about ten to almost fifteen minutes. For heavy soil and/or rust conditions, even up to about thirty minutes or so may be required.

For such preliminary cleaning treatment in an alkaline bath, and possibly more so for organic soils, it may be helpful to include an alkali metal carbonate in an amount below that at which its concentration could interfere with the quality of the resulting iron electrodeposit. Excess such carbonate even can prevent entirely the iron deposition. Present indications are that such carbonate content should be restricted to no more than about ten percent of the total dissolved solids in the bath.

For some soils, perhaps more so with oil and grease, it can help to include a small percentage, generally under one-half percent and possibly more often about half of that or less, of a synthetic detergent, nonionic or anionic and at times even cationic, or a mixture of any of thein, as specific conditions may dictate.

As stated earlier above, the iron electrodeposit needs to be sutficient to cover the initial ordinarily coarse, generally grainy surface of the particular cast iron article being plated. Because of the relatively rough and irregular surface of even the initially buffed and polished cast iron base, in that it is not fully flat and smooth as in the case of low carbon steel, no specific numerical minimum electrodeposit thickness can be given for the various articles which can be made of each different cast iron. However, the electrodeposit thickness appears to be adequate when the cleaned, buffed and polished surface is covered with the plated iron to the extent that the plated surface appears to be covered by a firmly adherent, overall substantially continuous electrodeposit of the iron, which can take the blue-black finish treatment without adverse effect.

Such adherent, overall substantially continuous minimum deposit then must be suflicient to resist flaking or being destroyed by the conditions prevailing in the blueing, i.e. blue-black iron oxide, producing treatment. Ordinarily, the iron electrodeposit thickness does not have to be much more than that just described above, even though the thus plated surface then may not be entirely fiat. Slightly thicker deposits even below 0.0001 inch could be more practical.

However, it is ditficult also to set a numerical maximum electrodeposit thickness applicable to all surfaces of the various cast iron articles. While about 0.0001 inch thick might be more than enough for most conditions, yet, where particularly needed or desired, it could be as much as up to about 0.0002 inch thick. Generally, there does not appear to be any particular need to plate a deposit thicker than that.

While the method of the invention is applicable to producing a blue-black iron oxide finish on articles made of any of the different cast irons, it is applicable particularly to electrodeposition of iron embraced in the process of producing a blue-black iron oxide finish on nodular iron. Accordingly, the invention will be more fully illustrated below, but is not intended to be restricted, by a description of details of its application to the production of such blue-black finish on nodular iron including the intermediate step of applying an adherent, overall substantially continuous electrodeposit of iron on the cleaned nodular iron articles such as receivers for rifles. The method of the invention applies equally to producing such a blue-black iron oxide finish on articles made of any other of the different cast irons.

Nodular iron castings of rifle receivers of relatively rectangular shape and roughly about 9 inches long by about 2.5 inches high by about 1.5 inches thick (with the customary bores and cavity) were cleaned of adhering loose mold sand, in customary manner. They then were precleaned by brushing With an organic solvent mixture of chlorinated aliphatic hydrocarbon and hydrocarbon solvents, followed by treatment with a heavy duty aqueous alkaline cleaner (containing soda ash, caustic soda, tetrapyrophosphate, and sodium metasilicate), and a warm water rinse.

The thus cleaned receivers, hung suitably spaced from one another from the horizontal arms of a cathode rack, were immersed, properly spaced about 8 inches from hard rolled steel anodes (having about 1.5 times the cathode area), in an aqueous alkaline iron plating bath held at F. and containing per gallon about 28.5 ounces of sodium hydroxide, about 7.05 ounces of sodium gluconate, about 1.15 ounces of sodium carbonate, 0.72 ounce of trisodium phosphate-(10 H 0), and about 3 ounces of ferric acetate.

While not critical to the electrodeposition, the bath also contained about 0.036 ounce of mixed wetting agents (4 parts anionic to 1 part anionic) and the same content of a sulfonated oil, viz. sulfonated butyl oleate.

To assure removal of any iron oxide possibly present on the exposed surfaces of the receivers, initially they were subjected in that bath to a reverse current cycle at 15 amperes per square foot for 3 minutes. Then without removing them from the bath, direct current (set to deliver 35 amperes per square foot of cathode area) was passed from the anodes to these receivers as cathodes to deposit iron on them for twenty-five minutes. The nodular iron receiver castings then were removed, rinsed adequately with warm water and allowed to air dry. They showed a continuous and uniform electrodeposit of iron firmly adhering to and completely covering the original speckle spotted nodular iron surface.

The thus iron plated receivers then were immersed in the customary bath used for the blueing treatment, to produce on their iron electrodeposited surfaces the blueblack iron oxide finish.

In the foregoing iron plating bath formulation, the various ingredients can be varied Within their respective ranges per gallon, namely, the sodium hydroxide from about 25.8 to about 32.25 ounces, the sodium gluconate from about 5.1 to about 8.9 ounces, the sodium carbonate from about 1 to about 1.3 ounces, the trisodium phosphate-(10 H O) from about 0.6 to about 0.8 ounce, the ferric acetate from about 2.2 to about 6.67 ounces and beneficially from about 3.88 to about 4.42 ounces, and the mixed wetting agents from about 0.03 to about 0.04 ounce, and so also the sulfonated oil.

For some baths, any or all of the trisodium phosphate, the wetting agents, and the sulfonated oil even may be omitted as they are not critical to the iron electrodeposition. However, each of them in its own way contributes to the smoothness of the plating operation, for example, at the respective anodes as well as to the rinsing off of adherent plating solution when the finished plated articles are removed from the bath. The ferric acetate can be replaced by iron equivalent amounts of ferric citrate, ferric gluconate, and the ferric chelate of any other carboxylic acid group containing sequestering agent which chelates iron at the pH of the bath.

Any of the various combined bath operating conditions in the foregoing more fully described illustrative operation can be changed at least within the various ranges disclosed herein to be practical. With this bath, it may be undesirable to operate below 170 F. and even better to work at at least about 175 F. and even better from 185 F. The current density might run from about 20 amperes per square foot with correspondingly longer plating timeand the 35 amperes per square foot generally should not be greatly exceeded. So also even at 35 amperes per square foot, the plating time will need to be increased for plating on a larger size receiver as to 45 minutes.

So also the same bath at any combination of suitable operating conditions can be used for cleaning and also to place a corresponding iron electrodeposit on any other castings of nodular iron or of any other of the various cast irons disclosed herein. As already stated above, the already described firmly adherent overall substantially continuous electrodeposit of iron can be plated out of any other suitable acqueous alkaline iron plating bath such as are illustrated by, but not restricted to, the acqueous compositions containing respectively the following:

Grams per liter Sodium glucoheptonate 45 Sodium hydroxide 185 Sodium carbonate a- 10 This bath shows pH 13.8. Its sodium carbonate content can be reduced or even omitted for it is not essential to obtaining from it a good iron electrodeposit. :In a mixture of dry ingredients, the sodium carbonate included serves to prevent caking of the mixture.

Grams per liter Citric acid 45 Sodium hydroxide 195 In this 'acqueous bath, obviously 28 grams of the sodium hydroxide neutralizes the citric acid so that the actual composition of the acqueous solution is equivalent in grams per liter to about 60 of sodium citrate and 167 of sodium hydroxide. It shows pH 13.6.

Grams per liter Sodium diethylentriamine pentaacetate 68 Sodium hydroxide 40 The sodium diethylenetriarnine pentaacetate used in making up this bath (of pH 13.6) was obtained and used as a 34% acqueous solution.

Grams Sodium glucoheptonate 0.3 Sorbitol 0.1 Sodium hydroxide 15 Water 84.6

The free alkali in any of the alkaline baths can be replaced by the equivalent or other amount of any other herein indicated applicable alkaline agent or even a compatible amine, e.g. diethylamine or a mono, d-1 or tri-ethanolamine or mixture of them. The quantity of any such alkaline agent in any of the foregoing baths,

or any modifications of them, can be varied by reasonable increase or decrease so long as the pH is above 11.

Likewise, the sequestering agent in any of those alkaline baths can be replaced in part or :as a whole by the same or other quantity of any other herein indicated two be applicable sequestering agents and within the indicated range so long as the bath still has a pH over 11.

The sodium citrate resulting from the neutralization in bath (b) can be added directly as sodium, or other alkali metal or ammonium, citrate Without any noticeable difference, or even by a compatible amine salt of it as indicated in the paragraph immediately following bath ((1) above.

The sodium diethylenetriamine pentaacetate can be replaced in whole or part by its corresponding salt of any other alkali metal or its ammonium or other aboveindicated amine salt, or by the corresponding tetrta-, tri-,di-, or mono-acetate, or even of ethylenetriarnine pentaacetic acid itself or by any other neutral or acid sequestering agent sufficiently soluble in water so long as enough sodium hydroxide or other earlier above disclosed alkaline agent is added if needed for the pH of the bath to be within the above noted range and about pH 11.

Thus, the hereinabove (page 11, line 14 to page 12, line 17) fully described illustrative operation, showing the overall treatment of nodular iron receivers, provides similarly practical results with its plating bath replaced by any of the other alkaline baths (a) through (d) above. That fully described illustrative operation then is to be considered as separately recorded here in full but with its alkaline bath replaced by any other alkaline bath respectively specifically identified in any of these foregoing specific alkaline baths or any above explained possible modifications of them, and including a watersoluble iron salt in the bath to facilitate initiating iron electrodeposition in it especially when that same bath is not used initially to clean the article to be plated. In such case, it is possible to include in alkaline plating bath an iron salt which while ordinarily insoluble or of limited solublity in water will dissolve in the bath by the chelating of its iron with the sequestering agent constituent of the bath.

Any water-soluble iron salt or chelate (e.g. ferrous gluconate) for the foregoing purpose advantageously can be included in a dry mix containing one or more such salt or chelate or other iron (ferrous or ferric) salt directly soluble or by chelation in the plating bath at its specific pH, along with the selected sequestering agent or agents, and also any alkaline substance which may be needed to adjust the bath to the required pH after the mix is dissolved in water. If cost were not a deterrent, the soluble form of iron and the required chelating agent or agents could be combined and included in the mix as the iron chelate equivalent of the quantity of sequestering agent content required.

More water-soluble iron salt can be included in such mix, up to a maximum which conveniently can be set at the amount of iron required to form the iron chelate with all of the sequestering agent content of the mix. However, it is advisable to keep the iron content of the mix below its chelate equivalent of all of its sequestering agent content.

The dry mixes are illustrated by, but not restricted to, the following compositions:

Grams Sodium glucoheptonate 45 Sodium hydroxide Ferrous gluconate 5 Sodium carbonate 10 This whole mix dissolved in a liter of water shows a pH of about that of bath (a) earlier above.

Grams Citric acid 45 Sodium hydroxide 195 Ferric citrate Sodium carbonate 10 240 grams of this mix dissolved in a liter of water has a pH about that of bath (b) above.

The ferrous gluconate or ferrous citrate in any of the dry mixes (e) and (f) can be increased or decreased within the hereinabove indicated range of content of iron in the dry mixes, or can be replaced in part or as a whole by the iron equivalent amount of the other of them or of any other ferrous or ferric chelate of any other chelating agent, such as those shown in any of the above-identified patents, or by any ferrous or ferric salt or salts soluble in the bath, e.g. ferric acetate.

Any of the sodium glucoheptonate or sodium hydroxide for the citric acid, in any of these dry mixes can be replaced in part or as a whole by an equivalent amount of any other alkali metal or ammonium salt or alkali metal hydroxide respectively or other hereinabove indicated salt of any of them or of any other applicable sequestering agent available in discrete particle form, with or without any minor amount of sodium or other alkali metal carbonate which may need to be added to prevent caking. All such dry mixes are embraced herein as part of this invention.

The nodular iron receivers in the earlier above fully described illustrative operation can be replaced by other nodular iron articles as well as by any articles composed of any other cast iron such as gray or white cast iron or malleable iron, and such other cast iron articles then similarly can be covered with an electrodeposit of iron and be subjected to a blueing treatment to be provided with a blue-black iron oxide finish.

So also the nodular iron receivers in said fully described illustrative operation can be replaced by a sintered compressed iron powder article, for example, gear wheels of 5.5 inches diameter by 1 inch thick. Thus, the procedure of cleaning such gear wheels, rinsing them, electroplating over them a firmly adherent, overall continuous electrodeposit of iron, warm water-rinsing and warm air drying them, and then subjecting them to a blueing treatment pro viding on them a blue-black iron oxide finish is included herein as if recited in the same full detail as given earlier above as to the nodular iron receivers, to avoid unduly extending this specification.

In preparing a plating bath by the use of an iron compound other than one which is a chelate, concern as to whether all of the iron of any iron hydroxide that could be precipitated by adding such iron compound to the strong alkali would be taken into solution readily by sequestration by the sequestering agent, it is advantageous to mix together the iron compound and the sequestering agent in water to form a uniform solution of the resulting iron chelate and then to mix that solution with the solution of the alkali.

While the invention has been explained by detailed description of certain specific embodiments of it, it is understood that various modifications and/or substitutions can be made in any of the embodiments within the scope of the appended claims which are intended also to cover equivalents of any of the embodiments.

What is claimed is:

1. The method of producing a blue-black iron oxide finish on a ferrous metal article member of the class consisting of a cast iron article and a sintered compressed iron powder article, which method comprises (a) cleaning from the exposed surfaces of said ferrous metal article any soil which could prevent electroplating on it a firmly adherent electrodeposit of iron;

(b) electroplating on said exposed surfaces a firmly adherent substantially continuous covering electrodeposit of iron while said ferrous metal is immersed as a cathode in an aqueous alkaline iron-plating bath maintained at a temperature of at least about F. and below that which would adversely affect the electrodeposit and having a pH of at least about 11 and dissolved therein (i) an amount of an iron compound to provide at least a sufiicient concentration of iron to enable initiating deposition of iron on said cathode on passing an electric current through said bath to said cathode as well as maintaining said iron deposition until the iron deposit on said cathode is sufiiciently thick to enable a satisfactory blue-black iron oxide finish thereafter to be produced on it,

(ii) an organic sequestering agent capable of sequestering iron at said pH and in a concentration sufiicient to prevent precipitation of any iron hydroxide under said plating conditions, and

(iii) an alkaline substance compatible with the other solutes in the bath and with the electrodeposit and in an amount sufiicient to maintain the pH at the selected level up to about 13.8; and

(c) continuing the electrodeposition until the iron deposit is sufficiently thick to enable said blue-black finish to be produced on it;

((1) removing the thus iron-plated ferrous metal article from said bath and any plating solution adhering to the electrodeposit; and

(e) thereafter producing on said electroplated surfaces a blue-black iron oxide finish.

2. A method as claimed in claim 1, wherein the sequestering agent is soluble in the bath, hydroxy-substituted lower aliphatic having up to seven carbon atoms and includes at least one group COOR wherein R is a cation which can be replaced by iron at whichever valence it exists in the bath and for it to form a chelate with the ligand portion of the sequestering agent.

3. A method as claimed in claim 2, wherein the sequestering agent is polyhydroxy.

4. A method as claimed in claim 3, wherein the sequestering agent content of the bath comprises primarily a compound having the formula and R is defined as in claim 2.

5. A method as claimed in claim 4, wherein the sequestering agent is an alkali metal gluconate.

6. A method as claimed in claim 5, wherein the alkali metal is sodium, and the alkaline substance is sodium hydroxide and the pH is from about 12.8 to about 13.8.

7. A method as claimed in claim 6, wherein the iron compound is ferric acetate.

8. A method as claimed in claim 7, wherein the anodes are iron-bearing.

9. A method as claimed in claim 6, wherein the ferrous metal article is formed of a cast iron.

10. A method as claimed in claim 9, wherein the cast iron articles are of nodular iron and the plating bath contains in ounces per gallon:

sodium hydroxide from about 25.8 to about 32.25,

sodium gluconate from about 5.1 to about 8.9, and

chelated iron at from about 0.53 to about 1.6 from said iron compound.

11. A method as claimed in claim 10, wherein the bath also contains:

sodium carbonate from about 1.0 to about 1.3,

trisodium phosphate (10 H O) from about 0.6 to about 0.8, and

said chelated iron is derived from ferric acetate as the iron compound at from about 2.2 to about 6.67 ounces per gallon of said bath.

12. A method as claimed in claim 10', wherein the temperature is from about 175 to about 195 F., the current density is from about 30 to about 50 amperes per square foot of cathode area, the electroplating is conducted for from about 30 to about 20 minutes, and the anode area is from about 1.5 times to about twice the cathode area.

13. A method as claimed in claim 6, wherein said ferrous metal article is a sintered compressed iron powder article.

14. A method as claimed in claim 13, wherein the plating bath contains in ounces per gallon:

sodium hydroxide from about 25.8 to about 32.25,

sodium gluconatc from about 5.1 to ab out 8.9, and chelated iron at from about 0.53 to about 1.6 from said iron compound.

References Cited JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner U.S. c1. X.R. 

